Make pkg/dart_messages strong.

pkg/dart_messages/bin/message_id.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:math' as math;
 
-import '../lib/shared_messages.dart' as shared_messages;
+import 'package:dart_messages/shared_messages.dart' as shared_messages;
 
 math.Random random = new math.Random();
 
 const idLength = 6;
 final $A = "A".codeUnitAt(0);
 final $Z = "Z".codeUnitAt(0);
 
 String computeId() {
-  List charCodes = [];
+  List<int> charCodes = [];
   for (int i = 0; i < idLength; i++) {
     charCodes.add($A + random.nextInt($Z - $A));
   }
   return new String.fromCharCodes(charCodes);
 }
 
 /// Computes a random message ID that hasn't been used before.
 void main() {
   var usedIds =
       shared_messages.MESSAGES.values.map((entry) => entry.id).toSet();
 
   print("${usedIds.length} existing ids");
 
   var newId;
   do {
     newId = computeId();
   } while (usedIds.contains(newId));
   print("Available id: $newId");
 }